Yes, it is art

Modern art has never been so popular - or so controversial. Fans of 'proper' sculpture and paintings 'that, you know, look like something' are outraged by Damien Hirst's pickled cows, Tracey Emin's filthy bed, Martin Creed's empty room. But the moaning won't change the facts, says Jonathan Jones: these are the direct descendants of Constable, Stubbs and Turner

Evolution has quickened to freakish speed, it seems, looking at the creature with multiple bodies, phalluses for noses and anuses for mouths that Jake and Dinos Chapman created in 1995 and named Zygotic Acceleration, Biogenetic, Desublimated Libidinal Model (Enlarged x 1000). Sometimes it feels as if hyper-mutation is what has happened to British art in the two decades since the Turner prize was invented; as if our art has been transformed with a brutal violence, by disruptive leaps, to become unrecognisable. The land of JMW Turner has become another country, another biological laboratory.

Yet the monsters have been with us for a long time, their eggs patiently nurtured in British culture. Their 19th-century grandparents can be found sulking among the shrubs in Crystal Palace Park, south London. Here, behind neat low fences, an iguanodon, with its horned nose, grimaces at the boating lake. An ichthyosaurus lies motionless in the water. They were made of brick and iron in 1854 and a banquet was held inside one of the hollow creatures, copious toasts made to the discoveries of the fossil hunters, the burgeoning empire of palaeontology. Today the dinosaurs in Crystal Palace Park are out of date, their anatomies reckoned inaccurate. And yet the monsters among the flowers constitute one of the most fascinating instances of public sculpture in Britain.

But, you may ask, is it art?

The question returns, naggingly, as you contemplate the innumerable dead flies of Damien Hirst's 1990 installation A Thousand Years. On a warm autumn day, it's a sad spectacle. Every single fly inside the divided glass tank that is supposed to contain myriads of buzzing insects, drawn from the chamber in which they are born into the one that contains a cow's head and an "insect-o-cutor", has passed away. No - one set of tiny legs twitches for a while, then stops. A big wet eyeball looks back at you. The cow's severed head is still gloved in soft brown fur; if the flies were alive they would have stripped it red by now. Instead of swarming there is stasis. This is a dead planet, silent and dry. The bottom of the tank, whose sides are yellow with fly shit, is a black shrivelled carpet. If A Thousand Years is an image of human history, or natural history, it has come to an end.

But is it art?

The question has hummed over the creations, collections and gestures of Damien Hirst and his contemporaries ever since Hirst was shortlisted for the Turner prize in 1992. When we ask, "is it art?" there is, implicitly, a contrast with the older British art that everyone knows to be art: with the paintings of Lucian Freud and Francis Bacon, of JMW Turner and John Constable, of Thomas Gainsborough and William Hogarth. It has been ritually mentioned as an irony that Turner, of all painterly painters, should have had his name hijacked by the prize that, since it was founded in the mid-1980s - and especially since it was relaunched in 1991 with an upper age limit of 50 - has become identified with all that is found, filmed, unlaundered, pickled and switched on and off in British art.

And yet the historical ground beneath our feet may not be as stable as we think. The great tradition of British painting is not the secure museum art we assume but a difficult, odd, eccentric phenomenon, never entirely understood or wholly liked by outsiders and, most interestingly of all, never considered simply as art. There always has been, in British art, a tendency to elide the aesthetic and the scientific, beauty and hard fact. Far from a violent departure, the art of the Turner prize era is a distillation of some of British art's most characteristic substances.